Many systems have been designed and proposed which intend to harvest the stronger and steadier winds hundreds of feet above the surface of the earth. Most propose using some sort of flying wind energy collection device connected to the ground by one or more tethers. These types of systems may be referred to as Airborne Wind Energy Collection Systems (AWECS). Patents have been granted on more than a dozen AWECS starting as far back as the 1800s. Two primary types of systems have been proposed. “Fly-Gen” systems carry a wind energy collection device and an electrical generator aloft, transmitting the produced electricity to the ground via an electrically conductive tether. “Ground-Gen” systems keep the generator on the ground and generally use the flying vehicle to pull the tether resulting in a spinning force being applied to the generator system on the ground.
Both types of systems have been proven to generate electricity, at least for a few seconds, by academic and/or commercial teams. However, no AWECS has ever become a commercially available product, primarily for technological reasons. Generally, prior art systems suffered from two primary technological disadvantages that made the most practical AWECS designs commercially infeasible. More specifically, the strength to weight ratios of tethers and the cost to performance ratio of automation systems were both woefully inadequate.
Twenty years ago, before the advent of extremely high strength to weight ratio synthetic fibers, AWECS with commercial levels of energy production simply were not possible. Steel cable would have been one of a few reasonably priced materials strong enough but disadvantaged if its weight would be more than a hundred pounds per thousand feet. That tether weight added to any flying vehicle's required lift made AWECS impractical. Additionally, steel cable utilized in a ground-gen system—likely performing more than a thousand power strokes every day and close to a million every year—would suffer extreme wear and need perhaps monthly replacement, an added maintenance cost making the system impractical to operate.
Additionally, automation systems and components, such as sensors, servos, and controllers were not developed or included in any meaningful way that met AWECS automation requirements, especially for ground-gen systems, to maintain system flight in the face of changing weather conditions, or to optimize power-stroke cycle of tether pulling and retraction, potentially thousands of times a day. In prior art AWECS, automated control systems did not facilitate commercial feasibility. No systems were known or implemented to allow the system to operate continuously, unattended by humans for weeks and months at a stretch.
Generally, AWECS development has been limited to research projects only. More recently, some of the AWECS known in the art may have some merit as commercially viable systems, but they still lack system requirements necessary for commercial success.
In particular known systems still lack launch and landing methods to get the flying vehicle into the air and back on the ground, automatically, reliably and repeatedly, with no human assistance.
No prior art for AWECS clearly defines this problematic part of the system. Yet even in the most ideal locations, some hours of every month will see winds drop below the speeds necessary to maintain unpowered flight. No known ground-gen AWEC systems define a method of maintaining flight in zero-speed winds aloft nor any control systems to control those flying vehicles to descend and land in such low-wind conditions. Similarly, no landing facility has been described in the prior art, nor any automated method of re-launching when wind conditions improve.
Prior art systems address the issue of system longevity with the assumption that those skilled in the art of designing flying vehicles will use materials light enough, strong enough, and sun and weather resistant enough for commercial feasibility. However, all prior art then describes systems made of fabrics proven by the ultralight aircraft industry to survive only a few thousand hours of exposure before becoming dangerously weakened. Similarly, prior art of ground-gen AWECS describes use of reels and/or capstans for the tether, with no recognition that more than 1000 power cycles every day is almost two orders of magnitude beyond what these standard line management systems handle in any other applications. Even the most recent high-tech ropes, wrapped around a capstan with a ton of force applied, cycled a few hundred feet back and forth, will shred from friction after a few thousand cycles. The prior art does not address longevity—the ability of the system to operate for many years with minimal down-time and replacement costs.
Some of the prior art describes systems requiring strength to weight ratios attainable only with the most advanced composite materials. These composites increase material costs high enough that the system purchase price then requires more than a decade of payback period—un-sellable in the renewable energy market. Alternatively, the prior art describes fabrics, as noted above, requiring replacement after only a few months of operation, thus increasing maintenance costs beyond commercially feasible levels. Similarly, high-friction capstan ground-gen systems may require tether replacements every few days, weeks or months, again inducing maintenance and replacement costs making the system commercially infeasible.
The prior art does not disclose or suggest broad flight envelopes—the ability for the system to maintain flight and energy production in the face of a broad range of wind and weather conditions. The prior art generally describes the flying vehicle in the most simple terms, leaving the details to those skilled in the art. The unmanned, autonomous wind-supported tethered flying structure required to operate in the broad range of wind speeds and weather required, has yet to be described. Unmanned Aerial Vehicles (UAV), some with a considerable amount of autonomy, exist today. UAV auto-pilot control systems are now commercially available off the shelf. However, tethered flight involves an entirely different set of stability algorithms. Increasing the complexity of the task is the requirement to have the AWECS remain aloft as long as possible—more than 90% of the time—by definition in wind and weather conditions UAVs will not risk operating within. Flight in wind speeds ranging from a gentle breeze to near hurricane force, and occasional gusts between the two extremes, is the real-world environment of commercially viable AWECS. Yet no prior art describes a flying vehicle able to cost-effectively maintain that broad a flight envelope.
The US Federal Aviation Administration (FAA) is an integral consideration in the design and deployment of any viable AWECS in the US. The FAA has defined rules concerning ‘moored’ flying objects, rules likely to be broadened to accept AWECS flying at higher altitudes and continuously. However, the requirement for making the moored object's tether visible to aircraft pilots is not likely to be dropped. On the contrary, with tether lengths many times the current allowable limits, making the tether highly visible to aircraft pilots will take on increased importance. Tethers on ground-gen AWECS are particularly problematic—experiencing more than a ton of load while extending and retracting possibly hundreds of feet with each power stroke. Flags directly mounted to the tether risk gumming up the works at the ground generation system. At a minimum, flags would require frequent replacement from the wear induced by such a torturous route. No prior art has described this critical piece of the AWECS puzzle.
Further, the high-strength tether lines AWECS require are particularly slippery. Slippery tethers are not a problem for fly-gen AWECS using just a reel. For ground-gen AWECS utilizing a capstan to capture the torque and motion of the tether, friction between the tether and the capstan is critical, but also damaging. Wrapped around a capstan perfectly functional for nylon or polyester lines, some lines will not only slip with the same number of coils, but with enough tension applied to spin the capstan, the coils will wrap atop one another. This coil to coil contact and resulting friction prematurely wears the tether, fraying and weakening it quickly enough to require replacement too frequently for commercial viability. The prior art makes no mention of, nor offers any solutions to the problems introduced by high-strength but slippery tether lines.